Before the decisive battle with Tatsuya Yuki, Reiji once again goes in search of combat practice, while Sei concentrates on fine-tuning the Build Strike. China is becoming concerned that Sei is too engrossed in his work. Then, a mysterious boy named Mao Yasaka appears before the two of them. Having won the Japanese 5th Block, he has already earned a place in the World Tournament, and he says he has come to the store because he is interested in Sei, a highly skilled builder who is the same age as he is. Mao has only one goal: to determine which of them is superior as a Gunpla builder. Sei accepts his challenge, and Sei and Mao bring out their Gunpla, the Build Strike and the Gundam X Maoh. As soon as they see each other's work, Sei and Mao find themselves adrift in space...!
Un jeune garçon du Kansai veut à tout prix rencontrer Sei pour comparer leurs niveaux. Il est vite perdu dans la ville, mais la rencontre finit par avoir lieu.
ユウキ・タツヤとの決戦前に、レイジは再び武者修行に出かけ、セイはビルドストライクの調整に余念がない。入れ込みすぎるセイを心配してやってくるチナ……そんな２人の前に、謎の少年、ヤサカ・マオが現れた。マオの目的は一つ、ビルダーとしてどちらのガンプラが優れているか、それを確かめたいのだ。勝負を受けたセイはビルドストライクを、マオがガンダムＸ魔王を取り出す。互いの作品を見つめるセイとマオは、直後、宇宙に投げ出されていた……！
与结城达也决战前夕，岭司再次出去锻炼技术，诚则专心调整创制突击高达。千奈担心诚会太过忘我，于是前来探望…此时一个名叫矢坂真生的神秘少年出现在他们眼前。真生已经在日本外围赛第五组出线，可晋级世界大赛。他对与自己同龄且同样拥有高超制作能力的诚很感兴趣，所以到访模型店。真生的目的别无其他，就是同为制作者，想弄清谁的高达模型更胜一筹。诚接受挑战并拿出创制突击高达，真生也拿出他的魔王X高达。两人凝视着对方的作品，下一刻却被抛出宇宙之中…
